I don't know how much bigger an 8 quart is than a 6, other than the obvious 2 quart volume. I got this one from Zulily (not there at the moment), and it fits into my 6 quart, but there's only about ½ to ¾ inch all around, and it comes up almost as high as the IP itself; on the original IP trivet, I can still get the lid on tightly. I'm not sure that's enough space for the IP to come to pressure, so I haven't given it a try, but your 8 may be able to handle this deep 3.5 quart bowl.
I haven't used it yet, but I just picked up this 3.5 qt bowl on Amazon for exactly this purpose. Reviews state that it fits in the 6qt Instant Pot and it's the largest single container that I could find confirmation of it being able to fit. It's twice the volume of the pyrex 7 cup bowl that the other comment mentioned.
https://smile.amazon.com/gp/product/B001C09S00/
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If you're okay with multiple containers, there's also something like this that can fit less per pot, but about the same/slightly more across the two containers (and gives you flexibility if you want to cook two things at once).
Bottom Pan: 6 cups 4 oz (52 oz)
Top Pan: 8 cups 4 oz (68 oz), but can fit up to 10 cups (80 oz) w/o the lid.
https://smile.amazon.com/gp/product/B07DDC34XY/
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